1. Field of the Invention
This invention relates generally to semiconductor integrated circuit devices having interconnects and more particularly, it relates to an improved metallization stack structure which has a higher electromigration resistance but yet maintains a relatively low resistivity of ultra large-scale-integration (ULSI) interconnects.
2. Description of the Prior Art
As is generally known to those skilled in the art, aluminum-based alloys have been used as an interconnection material in semiconductor integrated circuit devices for the past three decades. As the size dimensions of the integrated circuit devices are becoming smaller and smaller and decreasing down to a deep sub-micron range (i.e., 0.25 .mu.m and beyond) there has been raised a question as to the feasibility of using aluminum-based alloys as an interconnection material due to their two limiting factors. The first limiting factor of aluminum and aluminum-based alloys is because of a problem that disconnection or failure of the interconnection is liable to occur due to electromigration damage (EMD) and/or stress migration.
As defined herein, the term "electromigration" refers to a diffusion phenomenon which is based on an interaction between metal atoms in an interconnection (a metal line) and electrons moving in the interconnection. In particular, the electromigration is a phenomenon where the metal atoms migrate in the same direction as that of the electron movement. When this occurs, the resultant metal atom migration will cause an atomic vacancy or void to be formed at the location from which the metal atoms have moved, or will cause a hillock to be built up at the location where the metal atoms accumulate. When such voids are formed, the local cross-sectional area of the metal line will be decreased and the local current density in the metal line will be increased.
As explained in a paper article entitled "Copper Metallization for ULSI and Beyond" which was written by S. P. Murarka and S. W. Hymes, Solid State and Materials Sciences, Vol. 20, Issue 2, p. 87,1995, electromigration is caused by mass transport manifested as diffusion under a driving force. When a metal line is under the influence of an electric field and the current density is large enough (.gtoreq.10.sup.4 A/cm.sup.2), the current can displace the metal ions and move them away from their equilibrium positions. The enhanced and directional mobility of atoms are caused by (1) the direct influence of the electric field on ionized metal atoms, and (2) the collusion of electrons with atoms, leading to a momentum transfer (called electron-wind effect) and metal atom movement. The atomic flux moves in the direction of electron flow. At the same time, vacancies move in the opposite direction and form a vacancy flux. Whenever a gradient in temperature, grain size, geometric features, current density, and crystal orientation of grains, etc., occurs, metal atoms will be accumulated or depleted, forming hillocks or voids, which eventually grow large enough to cause metal line open or short failures.
The atomic flux due to electromigration can be expressed mathematically as follows: ##EQU1## where D is the atomic diffusivity N is the atomic density
Z is the effective charge on the moving ion PA1 o is the metal resistivity PA1 q is the electron charge PA1 j is the current density PA1 k is Boltzmann's constant PA1 T is temperature in degrees Kelvin
In Table I below, there is shown the electromigration parameters for the bulk materials of silver (Ag), aluminum (Al), gold (Au), and copper (Cu). As can be seen, the failures due to electromigration appear to be the worst in aluminum.
TABLE I __________________________________________________________________________ Electromigration parameters Dif.para. D(cm.sup.2 /s) Z*.rho.D(.mu..OMEGA.cm.sup.3 /s) Metal Z* .rho.(.mu..OMEGA.-cm) D.sub.0 (cm.sup.2 /s) O(eV) @100.degree. C. @100.degree. C. __________________________________________________________________________ Ag 9.4-23.4 1.6 1.89 2.01 1.1 .times. 10.sup.-26 2.84-7.0 .times. 10.sup.-25 Al 6.5-16.4 2.7 1.71 1.46 2.1 .times. 10.sup.-20 3.62-9.1 .times. 10.sup.-19 Au 5.9-7.4 2.4 0.67 1.96 2.2 .times. 10.sup.-27 3.05-3.8 .times. 10.sup.-26 Cu 3.7-4.3 1.7 0.78 2.19 2.1 .times. 10.sup.-30 1.3-15 .times. 10.sup.-29 __________________________________________________________________________
The second limiting fact or of aluminum and aluminum-based alloys is its resistivity. The pure aluminum has resistivity of about 2.7 .mu..OMEGA.-cm, which is significantly higher than that of pure copper (1.7 .mu..OMEGA.-cm). For practical integrated circuit applications, in order to increase its electromigration resistance, actual interconnects are made of aluminum alloys containing 0.5 to 2 weight-percent copper, which increases its resistivity to 3 to 3.5 .mu..OMEGA.-cm. In addition, the use of tungsten (W) plugs as vertical interconnects and titanium/titanium nitride as a barrier layer increases the effective final interconnect resistivity to as high as 4.5 to 5 .mu..OMEGA.-cm. Higher resistivity leads to higher RC delay (signal propagation delay) and limiting circuit speed.
In Table II below, there is shown a comparison of properties for different low-resistivity metals consisting of silver (Ag), aluminum (Al), gold (Au), copper (Cu), and tungsten (W). All the properties that must be considered in applications of these metals as interconnects in integrated circuits have been listed. As will be noted, silver offers the lowest resistivity, about 5% lower than that of copper. However, silver has poor electromigration resistance (see Table I). Also, silver does not adhere to silicon dioxide (SiO.sub.2) or other dielectrics, and it diffuses in the silicon dioxide at a much faster rate than copper (especially under bias).
TABLE II ______________________________________ Metals Property Cu Ag Au Al W ______________________________________ .rho.(.mu..OMEGA.-cm) 1.7 1.6 2.4 2.7 5.7 Melting point (.degree.C.) 1085 962 1064 660 3387 Atomic weight (amu) 64 108 197 27 184 TCR(.times.10.sup.3 /K) 4.3 4.1 4 4.5 4.8 TEC(.times.10.sup.3 /K) 17 19.1 14.2 23.5 4.5 Thermal conductivity(W/cm) 4.0 4.25 3.15 2.38 1.74 Special heat capacity(J/kg K) 386 234 132 917 138 Corrosion in air poor poor excellent good good Adhesion to SiO.sub.2 poor poor poor good poor Deposition sputtering yes yes yes yes yes evaporation yes yes yes yes yes CVD yes ? ? yes(?) yes Etching dry ? ? ? yes yes wet yes yes yes yes yes ______________________________________
In view of the foregoing, there has arisen a need for developing of new interconnection materials which will posses a higher degree of resistance to electromigration damage. From the above Tables I and II, it can be noted that copper is the best choice for use as interconnect material in integrated circuits. Copper has lower resistivity (about 30% lower than that of aluminum). In addition, the melting point for copper is more than 450.degree. C. higher and its atomic weight is more than two times heavier than that of aluminum. These properties indicate that copper will have a higher electromigration resistance than aluminum-based alloys. As a result, this would allow higher current density and narrower line width in copper interconnects, thereby improving current density and reducing interconnect resistance and capacitance, which will result in higher circuit speed.
Further, in a published article entitled "Electromigration-resistant Cu--Pd Alloy Films" which is authored by C. W. Park and R. W. Vook, Thin Solid Films, 226(1993), pp. 238-247, it is stated that copper is a prospective material for metal interconnects in microelectronic circuits since it has a high resistance to electromigration damage and a low electrical resistivity. Moreover, in order to achieve an even higher resistance to electromigration damage it was found that this could be accomplished by alloying copper (Cu) with a small amount of palladium (Pd). While the doped copper alloy exhibited a better electromigration reliability than pure copper, it suffered from the disadvantage of possessing a higher electrical resistivity than the pure copper.
Accordingly, there still exists a need of an interconnect structure which has a higher electromigration resistance but yet maintains a relatively low resistivity. The present invention represents a significant improvement over the Cu--Pd alloy structure discussed in the aforementioned Park et al. paper.